The influence of humic substances on the speciation and bioavailability ofdissolved mercury and methylmercury, measured as uptake by Chaoborus larvae and loss by volatilization

The influence of dissolved humic substances (HS) on the bioavailability of dissolved inorganic and methyl mercury (Mg) was quantified by measuring the direct uptake of Hg-203 in Chaoborus larvae using laboratory microcosms co ntaining artificial freshwater. The animals were exposed individually in tr iplicate aquaria at 10 different concentrations of HS covering the whole ra nge found in natural freshwaters (0-110 mg C l(-1)). Mercury-203 concentrat ions were monitored repeatedly in the same individuals and in their ambient water during up to 10 days. Near-steady state Hg concentrations in Chaobor us were usually reached within 5 days. The bioconcentration factor (BCF, di rect uptake only) for the larvae in the absence of HS was 0.55 +/- 0.09 (S. E.) mi individual(-1) for inorganic Hg and 5.3 +/- 0.7 mi individual(-1) fo r methyl Pig, thus showing a 10-fold difference. Normalizing to the organic carbon content of the larvae yields a BCFOC in the absence of HS of 2.8 +/ - 0.4 x 10(3) mi (gC)(-1) for inorganic Hg and 2.7 +/- 0.3 x 10(4) mi (gC)( -1) for methyl Hg. The uptake of both inorganic and methyl Hg decreased mar kedly with increasing concentration of WS. For inorganic Hg, the decrease i n uptake was most pronounced at HS concentrations below 0.2 mg C l(-1) For methyl Hg, the relationship between uptake and log([HS]) was sigmoid, showi ng a reduction by > 90% when increasing HS concentrations from 1 to 50 mg C l(-1). Similar patterns were observed for losses of Hg from the water phas e, mainly through volatilization. These results have implications for both the biouptake and the abiotic cycling of Hg in natural ecosystems and sugge st that most dissolved inorganic Hg is bound to dissolved organic matter in most natural freshwaters, whereas dissolved methyl Hg is bound only in hum ic waters. Assuming that only free aqueous Hg is taken up by the organisms, the rather simple methodology employed here can be used for estimating dis tribution coefficients (K-OC) for Hg between WS and water. In this study, t he K-OC values were 2.5 +/- 0.7 (S.E.)x 10(7) mi (gC)(-1) for inorganic Hg and 1.5 +/- 0.6 x 10(5) mi (gC)(-1) for methyl Hg. Values of similar magnit ude were derived from observed losses of Hg from the water phase, supportin g the assumption of an immobilization of both inorganic and methyl Hg by HS . The strong negative influence of dissolved HS on the bioavailability of b oth inorganic and methyl Hg in freshwater suggests that the high Hg levels often found in fish from humic lakes in the boreal forest zone cannot be ex plained alone by direct uptake of methyl Hg from the water phase into biota at low trophic levels. (C) 2000 Elsevier Science B.V. All rights reserved.